1. Field of the Invention
The present invention relates to steel strip annealing apparatuses, and more particularly to a steel strip continuous annealing apparatus comprising a heating zone, a soaking zone and cooling zones.
2. Description of the Prior Art
Recently, annealing processes for rendering predetermined processability, deep drawing properties and the like to cold-rolled steel strips have been carried out by continuous annealing apparatuses. These continuous annealing apparatuses are formed into peculiar predetermined configurations depending upon grades of steel, thickness of sheet, temperatures for heating and soaking, cooling conditions and the like.
More specifically, a continuous annealing apparatus for producing black tinplates, for example, has such a function that, in which, a steel strip having a sheet thickness of 0.15 to 0.6 mm and a sheet width of 600 to 1000 mm is soaked at a temperature of 700.degree. to 800.degree. C., thereafter, slowly cooled to about 450.degree. C. from the temperature described above without rapidly cooling, and further, rapidly cooled to 100.degree. C. to substantially room temperature, where the steel strip is not oxidized, outside the furnace, and consequently, the continuous annealing apparatus comprises a heating, a soaking, a slowly cooling and a rapidly cooling zones. In contrast thereto, a continuous annealing apparatus for producing cold-rolled steel sheets for drawing or soft black tinplates has such a function that, in which, a steel strip for producing cold-rolled steel sheet for drawing having a sheet thickness of 0.4 to 1.6 mm and a sheet width of 800 to 1500 mm or a steel strip for producing a soft black tinplate having a sheet thickness of 0.15 to 0.6 mm and a sheet width of 600 to 1000 mm is soaked to a temperature of 700.degree. to 850.degree. C., thereafter, rapidly cooled to a temperature of 300.degree. to 500.degree. C. at a cooling rate of approximately 10.degree. to 100.degree. C./sec, subjected to an overaging treatment being held at the temperature of 300.degree. to 500.degree. C. for 1 to 5 min so as to be satisfactorily softened, and then, rapidly cooled, and consequently, the continuous annealing apparatus comprises a heating, a soaking, a rapidly cooling, an overaging and a final cooling zones. Furthermore, a continuous annealing apparatus for producing high-strength cold-rolled steel sheet having a mixed structure has such a function that, in which, a steel strip having a sheet thickness and a sheet width similar to those of the cold-rolled steel sheet for drawing as described above is heated to a temperature of 800.degree. to 850.degree. C., caused to partially generate .gamma. phase in a ferrite structure, rapidly cooled at a cooling rate of approximately 10.degree. to 100.degree. C./sec, and turned into a product as it is. Further, a continuous annealing apparatus for producing silicon steel sheets has such a function that, in which, a steel sheet having a sheet thickness of 0.3 to 0.7 mm and a sheet width of 600 to 1000 mm can be heated to a comparatively high temperature of 800.degree. to 1000.degree. C. and soaked, and thereafter, cooled to substantially to room temperature without rapidly cooling, and consequently, comprises a heating zone, a soaking zone and a cooling zone. As has been described hereinabove, each continuous annealing apparatus is required to have a peculiar heat cycle depending on the material quality of the steel sheet to be annealed and a peculiar configuration depending on the dimensions of the steel sheet, and, it is difficult to treat in one and the same continuous annealing apparatus the black tinplates, cold-rolled steel sheet for drawing, soft black tinplates, high-strength cold-rolled steel sheets, silicon steel sheets or the like, which are different in required heat cycle and dimensions.
However, it is uneconomical to set the rate of conveying the steel strip at an excessively small value in a continuous annealing apparatus because the continuous annealing apparatuses each have a production capacity of 20,000 to 40,000 t/mon. on the average. Consequently, it is apparently inadvisable for an enterprise having an amount of customer's demand sufficient to constantly and sufficiently operate the respective types of continuous annealing apparatuses to possess the abovedescribed continuous annealing apparatuses meeting the conditions required for the types of steel strips. From the viewpoint as described above, necessity has been voiced for making it possible to selectively treat the black tinplates, cold-rolled steel sheets, high-strength cold-rolled steel sheets or silicon steel sheets in a single continuous annealing apparatus.
However, in the case of making it possible to selectively treat different types of steel sheets in the single continuous annealing apparatus, the steel strips for producing the soft black tinplates, cold-rolled steel sheets for drawing, high-strength cold-rolled steel sheets or the like are required to be rapidly cooled at a high rapidly cooling rate after being heated and soaked on one hand, and the steel strips for producing the black tinplates, silicon steel sheets or the like are required to be conveyed through the rapidly cooling zone at a soaking temperature on the other hand.
Here, as shown in FIGS. 1 and 2, a rapidly cooling zone 1 incorporates a plenum chamber 2 for forming means of forcible cooling, so that the steel strip being conveyed can be rapidly cooled at a predetermined cooling rate by cooling gas blown out of blow-out nozzles 3. The plenum chamber 2 and the blow-out nozzles 3 thereof are not raised in temperature during rapidly cooling of the steel strip because the cooling gas flows therethrough. However, when the steel strip is conveyed in the soaked condition through the rapidly cooling zone 1, as shown in FIG. 3, a difference in temperature between a point A on the outer surface of the plenum chamber 2 and a point B on the rear surface thereof becomes large for several to ten-odd minutes after the beginning of operation due to the radiant heat emitted from the steel strip being at high temperature, and an unbalance in stress in generated in the plenum chamber 2, whereby thermal deformation is caused to the plenum chamber 2. Further, as time goes by after the beginning of operation, the temperature of the plenum chamber 2 reaches substantially the same temperature as the temperature of the steel strip, and, when the temperature of the plenum chamber 2 is maintained high for a long period of time, the deformation of the plenum chamber by gravity progresses. In the case the deformation of the plenum chamber 2 as described above takes place, the distribution in flow rate of the cooling gas blown out of the blow-out nozzles 3 of the plenum chamber 2 changes for the worse, whereby the cooling power to the steel strip is varied to cause irregularities in cooling to the steel strip, thus presenting a problem of resulting in irregular shapes, cooling buckling and the like of the steel strip.